Even if the debris spotted floating 2,500 kilometres off the Australian coastline belongs to the missing Malaysian Airlines plane, the mystery of what happened on doomed flight MH370 might never be unlocked.

That's the warning from the world's leading crash investigation bureau - which investigated the 2009 crash of Air France flight 447 - for the 29 planes, 18 ships and six helicopters currently involved in the painstaking search of 600,000 square-kilometres of ocean.

It would mean the wait is far from over for the heartbroken families of the passengers aboard the Malaysian flight.

Rémi Jouty - head of France's Air Accident Investigation Bureau - said it might take years to recover the plane’s black box, if it can be recovered at all.

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The head of the French air crash investigation bureau, Rémi Jouty, (left) has warned that the mystery of doomed flight MH370 might take years to unlock, with search parties (right) yet to find the plane's black box

The search for debris thought to belong to flight MH370 has now resumed, with 29 planes, 18 ships and six helicopters involved

‘The only thing I can say is it will be most difficult and the recovery is not guaranteed,’ Remi Jouty, told the Financial Times.

The French bureau has dispatched a three-strong team of investigators to Malaysia to assist local authorities in retrieving the plane's black box recorder, which records sound along with logging the flight's data, and would unlock the mystery of flight MH370.

The Royal Australian Air Force are dropping marker buoys to measure currents and tides to help track the wreckage

The search is focused 2,500km off Australia's west coast, where satellite images revealed two pieces of debris floating in the open ocean

All three were involved in the investigation into the crash of Air France flight 447, which went down in the South Atlantic. Mr Jouty said despite finding that wreckage within 24 hours of the crash, it took officials two years to retrieve the flight’s black boxes from the ocean floor.

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The devices were eventually recovered from a depth of 4,000 metres – approximately the same depth as the section of the Indian Ocean where search parties are working to locate debris believed to belong to flight MH370.

Search teams working to recover the debris from the 2009 Air France crash

It took investigators two years to recover the black box recorder from the Air France crash

Mr Jouty said the 2009 investigation was hampered by the size of the search area, with French authorities unable to locate the recorders despite having a relatively sure idea of where they might be.

But the search area for MH370 will be substantially larger, given the debris may have been drifting since the flight's disappearance on March 8, and there’s no way of knowing whether the plane’s black boxes are anywhere near the rest of the wreckage.

Investigator Alain Bouillard, who headed the French bureau’s 2009 search, told the Sydney Morning Herald that the search MH370 would be ‘far, far harder than what we had looking for Air France 447.’

‘Firstly we had many more clues,' he said. 'We knew that the Air France plane had a problem, thanks to 24 ACAR messages sent over four minutes; we knew its precise location four minutes before impact, which allowed us to reduce our search zone to only 40 nautical miles.

‘That is nothing compared to the surface area of today's search.’

The satellite image of objects thought to be debris from the missing Malaysia Airlines Flight MH370

A Royal Australian Air Force P3 Orion takes off from Pearce air base to help continue the search

The 63-year-old said merely finding the debris would be the first step in a long investigation into locating the plane's black box recorder, which could be up to 150 miles away from the wreckage.

'Objects that have drifted for two weeks will have traveled a long way in that time,' he said.

'if you have currents at four knots, that mean four nautical miles per day and a considerable distance in 14 days.'

But the crash expert said the debris would at least begin to paint a picture of what happened aboard Malaysian flight MH370.

'After you have identified and examined some debris, you can piece together how the plane broke up,' he said. 'Was it in the air, was it during a sea landing, or did it hit the ocean surface? From that you can build up a scenario.'